Signal processing circuit for electrostatic capacity type touch sensor

ABSTRACT

There is offered a signal processing circuit for an electrostatic capacity type touch sensor which can improve the noise tolerance and adjust an offset in the output voltage. The signal processing circuit for the touch sensor is structured to include an alternating current power supply providing an excitation pad with an alternating voltage, an electric charge amplifier generating an output voltage Vout corresponding to a difference between a capacitance of a first capacitor formed between a first touch pad and the excitation pad and a capacitance of a second capacitor formed between a second touch pad and the excitation pad, and an offset adjustment circuit to adjust an offset in the output voltage Vout of the electric charge amplifier.

CROSS-REFERENCE OF THE INVENTION

This application claims priority from Japanese Patent Application No.2009-139229, the content of which is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a signal processing circuit for anelectrostatic capacity type touch sensor.

2. Description of the Related Art

The electrostatic capacity type touch sensor has been known as an inputdevice to various kinds of electronic devices such as a mobile phone, aportable audio device, a portable game console, a television and apersonal computer.

A conventional electrostatic capacity type touch sensor is explainedreferring to FIG. 10 and FIG. 11. A touch pad 61 is formed on a PCB(Printed Circuit Board) substrate 60 so that an electrostatic capacitor62 (capacitance C) is formed between the touch pad 61 and the PCBsubstrate 60, as shown in FIG. 10. The touch pad 61 is connected to anon-inverting input terminal (+) of a comparator 63 through a wiring 64.A reference voltage Vref is applied to an inverting input terminal (−)of the comparator 63. A constant current power supply 65 is connected tothe wiring 64 that connects the touch pad 61 and the non-inverting inputterminal (+) of the comparator 63.

Operations of the electrostatic capacity type touch sensor are explainedreferring to FIG. 11. When a finger 66 of an operator is far away fromthe touch pad 61, a capacitance associated with the touch pad 61 is C.In this case, a voltage at the touch pad 61 increases from 0 V in areset state as the electrostatic capacitor 62 is charged with a constantcurrent from the constant current power supply 65. An output voltage ofthe comparator 63 is inverted when the voltage at the touch pad 61reaches the reference voltage Vref. A length of time from the resetstate to the inversion of the comparator 63 in this case is referred toas t1.

When the finger 66 of the operator approaches the touch pad 61, on theother hand, the capacitance associated with the touch pad 61 increasesto C+C′. The increment C′ is a capacitance of a capacitor formed betweenthe finger 66 of the operator and the touch pad 61. As a result, thelength of time that the voltage at the touch pad 61 takes from 0 V tothe reference voltage Vref increases to t2 (t2>t1). That is, it ispossible to detect whether the finger 66 of the operator has touched thetouch pad 61 or not, based on a difference (t2−41) in the length of timetaken by the transition from the reset state to the inversion of thecomparator 63. In other words, the touch pad 61 functions as an ON/OFFswitch for data input.

Technologies mentioned above are disclosed in Japanese PatentApplication Publication No. 2005-190950, for example.

In the conventional touch sensor, however, there is a problem that thevoltage at the touch pad 61 is varied to cause malfunctioning when anoise is applied to the touch pad 61.

SUMMARY OF THE INVENTION

The invention provides a signal processing circuit for an electrostaticcapacity type touch sensor that receives a signal from a first andsecond touch pads disposed on a touch panel using an excitation paddisposed between the first and second touch pads for signal processing.The signal processing circuit includes a first alternating current powersupply generating a first alternating voltage, and an electric chargeamplifier generating an output voltage corresponding to a differencebetween a first capacitance of a first capacitor and a secondcapacitance of a second capacitor when the first alternating voltage isapplied to the excitation pad. The first capacitor is formed between thefirst touch pad and the excitation pad, and the second capacitor isformed between the second touch pad and the excitation pad. The signalprocessing circuit also includes an offset adjustment circuit adjustingan offset in the output voltage of the electric charge amplifier.

The invention provides a signal processing device that includes a touchpanel and a signal processing circuit for the touch panel. The touchpanel includes a plurality of pairs of touch pads, each of the touchpads is a first kind of touch pad, a second kind of touch pad, a thirdkind of touch pad or a fourth kind of touch pad, and each of the pairsof touch pads includes two touch pads of a single kind or two touch padsof two different kinds. The touch panel also includes an excitation paddisposed between neighboring two of the pairs of touch pads. The signalprocessing circuit includes a first alternating current power supplyproviding the excitation pad with a first alternating voltage, anelectric charge amplifier generating a first output voltagecorresponding to a difference between a capacitance of a first capacitorformed between the excitation pad and the first kind of touch pad and acapacitance of a second capacitor formed between the excitation pad andthe second kind of touch pad and generating a second output voltagecorresponding to a difference between a capacitance of a third capacitorformed between the excitation pad and the third kind of touch pad and acapacitance of a fourth capacitor formed between the excitation pad andthe fourth kind of touch pad, and an offset adjustment circuit adjustingan offset in the first output voltage or an offset in the second outputvoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C show an electrostatic capacity type touch sensor anda signal processing circuit according to a first embodiment of thisinvention.

FIG. 2 shows input/output characteristics of an electric chargeamplifier.

FIG. 3 is a circuit diagram of the electric charge amplifier.

FIGS. 4A and 4B show operations of the electric charge amplifier.

FIG. 5 shows an electrostatic capacity type touch sensor and a signalprocessing circuit according to a second embodiment of this invention.

FIG. 6 shows waveforms of outputs of the signal processing circuit forthe electrostatic capacitor type touch sensor.

FIG. 7 shows correlation between output voltages of the electric chargeamplifier and an angle θ of a touch position.

FIG. 8 shows an electrostatic capacity type touch sensor and a signalprocessing circuit according to a third embodiment of this invention.

FIG. 9 shows a system structure of the electrostatic capacity type touchsensor and the signal processing circuit.

FIG. 10 shows a conventional electrostatic capacity type touch sensor.

FIG. 11 is to explain operations of the conventional electrostaticcapacity type touch sensor.

DETAILED DESCRIPTION OF THE INVENTION

A signal processing circuit for an electrostatic capacity type touchsensor according to a first embodiment of this invention is hereafterdescribed referring to the drawings.

The touch sensor is structured so that an excitation pad 12, a firsttouch pad 13 and a second touch pad 14 are disposed on a substrate 11such as a PCB substrate in a way that the excitation pad 12 isinterposed between the first and second touch pads 13 and 14, as shownin FIGS. 1A, 1B and 1C. A dielectric layer (not shown) is formed betweenthe excitation pad 12 and each of the first and second touch pads 13 and14.

That is, a first electrostatic capacitor C1 is formed of the excitationpad 12 and the first touch pad 13. Similarly, a second electrostaticcapacitor C2 is formed of the excitation pad 12 and the second touch pad14. The first electrostatic capacitor C1 has a capacitance CA1, whilethe second electrostatic capacitor C2 has a capacitance CA2. It ispreferable that the capacitances CA1 and CA2 are set to be equal to eachother in an initial state. Since the excitation pad 12 and the first andsecond touch pads 13 and 14 are electrodes, it is preferable thatsurfaces of these electrodes are covered with an insulator such asplastic, wood or rubber.

On the other hand, an alternating current power supply 16 that providesthe excitation pad 12 with an alternating voltage through a wiring 15 isprovided on a side of the signal processing circuit (IC). Amplitude ofthe alternating voltage is referred to as an excitation voltage Vref.Also, there is provided an electric charge amplifier 17. The first touchpad 13 is connected to a non-inverting input terminal (+) of theelectric charge amplifier 17 through an wiring 18, while the secondtouch pad 14 is connected to an inverting input terminal (−) of theelectric charge amplifier 17 through a wiring 19.

The electric charge amplifier 17 generates a voltage corresponding to adifference between the capacitance CA1 of the first capacitor C1 formedbetween the excitation pad 12 and the first touch pad 13 and thecapacitance CA2 of the second capacitor C2 formed between the excitationpad 12 and the second touch pad 14.

An example of a concrete structure of the electric charge amplifier 17is described hereafter referring to FIG. 3, FIG. 4A and FIG. 4B. Thefirst capacitor C1 and the second capacitor C2 are formed on thesubstrate 11 that is depicted as a portion encircled by a dashed line inFIG. 3. Portions of the structure shown in FIG. 3 except for thesubstrate 11 make the signal processing circuit.

The alternating current power supply 16 is formed of switches SW1 andSW2 that are switched alternately. The alternating current power supply16 outputs a ground voltage (0 V) when the switch SW1 is closed and theswitch SW2 is opened, and outputs the excitation voltage Vref (positivevoltage) when the switch SW1 is opened and the switch SW2 is closed. Inthis case, the alternating current power supply 16 outputs a clocksignal voltage alternating between the excitation voltage Vref (H level)and 0 V (L level).

A third electrostatic capacitor C3 is connected in series with the firstelectrostatic capacitor C1, while a fourth electrostatic capacitor C4 isconnected in series with the second electrostatic capacitor C2. Thethird electrostatic capacitor C3 has a capacitance CA3, while the fourthelectrostatic capacitor C4 has a capacitance CA4.

An alternating current power supply 21, that is similar to thealternating current power supply 16, is connected to a connecting nodebetween the third and fourth electrostatic capacitors C3 and C4. Thealternating current power supply 21 is formed of switches SW3 and SW4that are switched alternately. The alternating current power supply 21outputs the ground voltage (0 V) when the switch SW3 is closed and theswitch SW4 is opened, and outputs the excitation voltage Vref (positivevoltage) when the switch SW3 is opened and the switch SW4 is closed. Thealternating current power supply 16 and the alternating current powersupply 21 output the clock signal voltages that are opposite in phase toeach other.

A wiring drawn out from a connecting node N2 between the first and thirdelectrostatic capacitors C1 and C3 is connected to a non-inverting inputterminal (+) of an ordinary differential amplifier 22, while a wiringdrawn out from a connecting node N1 between the second and fourthelectrostatic capacitors C2 and C4 is connected to an inverting inputterminal (−) of the differential amplifier 22.

A feedback capacitor Cf is connected between an inverting outputterminal (−) and the non-inverting input terminal (+) of thedifferential amplifier 22, while an identical feedback capacitor Cf isconnected between a non-inverting output terminal (+) and the invertinginput terminal (−) of the differential amplifier 22. Each of thefeedback capacitors Cf has a capacitance CAf.

A switch SW5 is connected between the inverting output terminal (−) andthe non-inverting input terminal (+) of the differential amplifier 22,while a switch SW6 is connected between the non-inverting outputterminal (+) and the inverting input terminal (−) of the differentialamplifier 22. The switches SW5 and SW6 are switched simultaneously. Thatis, when the switches SW5 and SW6 are closed, the inverting outputterminal (−) and the non-inverting input terminal (+) of thedifferential amplifier 22 are short-circuited while the non-invertingoutput terminal (+) and the inverting input terminal (−) of thedifferential amplifier 22 are short-circuited.

A voltage difference between an output voltage Vom from the invertingoutput terminal (−) of the differential amplifier 22 and an outputvoltage Vop from the non-inverting output terminal (+) of thedifferential amplifier 22 is represented by an output voltage Vout(=Vop−Vom).

In the structure described above, the third and fourth electrostaticcapacitors C3 and C4 are formed of variable capacitance capacitors inorder to compensate an offset in the output voltage Vout due to animbalance between the first and second electrostatic capacitors C1 andC2. That is, the third electrostatic capacitor C3 is formed to include melectrostatic capacitors C₃₁-C_(3m), each having each of capacitancesCA₃₁-CA_(3m), respectively, and m switches S₃₁-S_(3m). It is preferablethat the capacitances CA₃₁-CA_(3m), are weighted so that the capacitanceof the third electrostatic capacitor C3 is minutely modified. Forexample, when the capacitance CA₃₁ of the capacitor C₃₁ is denoted asC₀, CA₃₂=½C₀, CA₃₃=¼ C₀, CA₃₄=⅛ C₀, CA_(3m)=½^(m−1) C₀. Each of theswitches S₃₁-S_(3m) is connected between corresponding each of theelectrostatic capacitors C₃₁-C_(3m) and the alternating current powersupply 21. Each of the switches S₃₁-S_(3m) is turned on and off bycorresponding each of m-bits of adjustment signals from an offsetadjustment circuit 42. That is, when a switch S_(3x), that is one of theswitches S₃₁-S_(3m), is turned on, an electrostatic capacitor C_(3x),that is corresponding one of the capacitors C₃₁-C_(3m), is electricallyconnected between the first electrostatic capacitor C1 and thealternating power supply 21.

Also, the fourth electrostatic capacitor C4 is formed to include melectrostatic capacitors C₄₁-C_(4m), each having each of capacitancesCA₄₁-CA_(4m), respectively, and m switches S₄₁-S_(4m). It is preferablethat the capacitances CA₄₁-CA_(4m) are weighted for the same reason. Forexample, when the capacitance CA₄₁ of the capacitor C₄₁ is denoted asC₀, CA₄₂=½ C₀, CA₄₃=¼ C₀, CA₄₄=⅛ C₀, CA_(4m)=½^(m−1) C₀. Each of theswitches S₄₁-S_(4m) is connected between corresponding each of theelectrostatic capacitors C₄₁-C_(4m) and the alternating current powersupply 21. Each of the switches S₄₁-S_(4m) is turned on and off bycorresponding each of m-bits of adjustment signals from the offsetadjustment circuit 42. That is, when a switch S_(4x), that is one of theswitches S₄₁-S_(4m), is turned on, an electrostatic capacitor C_(4x),that is corresponding one of the capacitors C₄₁-C_(4m), is electricallyconnected between the second electrostatic capacitor C2 and thealternating power supply 21.

With the structure described above, the capacitances CA3 and CA4 of thethird and fourth capacitors C3 and C4 can be adjusted by thecorresponding m-bits of adjustment signals from the offset adjustmentcircuit 42. The offset adjustment circuit 42 can determine the 2m-bitsof adjustment signals with which the offset in the output voltage Voutbecomes a desired value, that is preferably the minimum value, based onthe output voltage Vout. The determined adjustment signals are writteninto an electrically writable/erasable non-volatile memory such as anEEPROM 43 by a control circuit and stored.

Next, operations of the circuit structured as described above will beexplained referring to FIGS. 4A and 4B. Each of the third and fourthelectrostatic capacitors C3 and C4 is represented as a symbol of singlecapacitor in FIGS. 4A and 4B, for the sake of simplicity. The circuithas a charge accumulation mode and a charge transfer mode that alternatea multitude of times.

In the charge accumulation mode that is shown in FIG. 4A, the excitationvoltage Vref is applied to the first and second electrostatic capacitorsC1 and C2 by opening SW1 and closing SW2 of the alternating currentpower supply 16. Also the ground voltage (0 V) is applied to the thirdand fourth electrostatic capacitors C3 and C4 by opening SW4 and closingSW3 of the alternating current power supply 21.

Also, SW5 and SW6 are closed. With this, the inverting output terminal(−) and the non-inverting input terminal (+) of the differentialamplifier 22 are short-circuited while the non-inverting output terminal(+) and the inverting input terminal (−) are short-circuited. As aresult, a voltage at the node N1 (node of the wiring connected to theinverting input terminal (−)), a voltage at the node N2 (node of thewiring connected to the non-inverting input terminal (+)), a voltage atthe inverting output terminal (−) and a voltage at the non-invertingoutput terminal (+) all become ½ Vref. Here, a common mode voltage ofthe differential amplifier 22 is ½ Vref, which is a half of theexcitation voltage Vref.

Next, in the charge transfer mode that is shown in FIG. 4B, the groundvoltage (0 V) is applied to the first and second electrostaticcapacitors C1 and C2 by closing SW1 and opening SW2 of the alternatingcurrent power supply 16. Also, the excitation voltage Vref is applied tothe third and fourth electrostatic capacitors C3 and C4 by closing SW4and opening SW3 of the alternating current power supply 21. Also, SW5and SW6 are opened.

After that, the circuit returns to the charge accumulation mode shown inFIG. 4A, and then turns to the charge transfer mode again. The electriccharge amplifier 17 reaches a stable state after repeating theoperations described above a multitude of times.

In the charge accumulation mode,

$\begin{matrix}{{{Amount}\mspace{14mu} {of}\mspace{14mu} {Electric}\mspace{14mu} {Charges}\mspace{14mu} {at}\mspace{14mu} N\; 1} = {{{CA}\; {2 \cdot \left( {- \frac{Vref}{2}} \right)}} + {{CA}\; {4 \cdot \left( \frac{Vref}{2} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

where CA2·(−½ Vref) represents an amount of electric charges stored inC2, and CA4·(½ Vref) represents an amount of electric charges stored inC4.

In the charge transfer mode,

$\begin{matrix}{{{Amount}\mspace{14mu} {of}\mspace{14mu} {Electric}\mspace{14mu} {Charges}\mspace{14mu} {at}\mspace{14mu} N\; 1} = {{{CA}\; {2 \cdot \left( \frac{Vref}{2} \right)}} + {{CA}\; {4 \cdot \left( {- \frac{Vref}{2}} \right)}} + {{CAf} \cdot \left( {{Vop} - \frac{Vref}{2}} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

where CA2·(½ Vref) represents an amount of electric charges stored inC2, CA4·(−½ Vref) represents an amount of electric charges stored in C4,and CAf·(Vop−½ Vref) represents an amount of electric charges stored inCf. [Equation 1]=[Equation 2] holds, since the amount of electriccharges at the node N1 in the charge accumulation mode is equal to theamount of electric charges at the node N1 in the charge transfer modeaccording to the law of conservation of electric charge.

Following equation is obtained by solving [Equation 1]=[Equation 2] forVop.

$\begin{matrix}{{Vop} = {\left( {1 - \frac{{CA}\; 2}{CAf} + \frac{{CA}\; 4}{CAf}} \right) \cdot \left( \frac{Vref}{2} \right)}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Similarly, following equation is obtained by applying the law ofconservation of electric charge to the electric charges at the node N2and solving the resulting equation for Vom.

$\begin{matrix}{{Vom} = {\left( {1 - \frac{{CA}\; 1}{CAf} + \frac{{CA}\; 3}{CAf}} \right) \cdot \left( \frac{Vref}{2} \right)}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Vout is obtained from [Equation 3] and [Equation 4].

$\begin{matrix}{{Vout} = {\frac{{{CA}\; 1} - {{CA}\; 2} - \left( {{{CA}\; 3} - {{CA}\; 4}} \right)}{CAf} \cdot {Vref}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

The capacitances CA1 and CA2 of the electrostatic capacitors C1 and C2,each formed between the excitation pad 12 and each of the first andsecond touch pads 13 and 14, respectively, are set to be equal to eachother, and are set to be CA1=CA2=CA3=CA4=C in the initial state in whichthe finger 20 of the operator is far away from the touch pads 13 and 14.When the finger of the operator approaches to the touch pad, acapacitance difference ΔC is caused between CA1 and CA2. That is,CA1−CA2=ΔC.

In this case, the output voltage Vout is represented by the followingequation as derived from the equation 5.

$\begin{matrix}{{Vout} = {\frac{\Delta \; C}{CAf} \cdot {Vref}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

That is, it is understood that the output voltage Vout of the electriccharge amplifier 17 varies proportionally to the capacitance differenceΔC between the capacitances CA1 and CA2, as shown in FIG. 2.

In reality, however, it is difficult to set the capacitances CA1 and CA2of the electrostatic capacitors C1 and C2, each formed between theexcitation pad 12 and each of the first and second touch pads 13 and 14,respectively, perfectly equal to each other because of an influence ofparasitic capacitances of wirings or the like connected to the touchpads 13 and 14. As a result, there exists a capacitance difference ΔC'between CA1 and CA2 even in the initial state. In this case,CA1−CA2=ΔC′. Assuming CA3=CA4=C, the output voltage Vout is representedby the following equation as derived from the equation 5.

$\begin{matrix}{{Vout} = {\frac{\Delta \; C^{\prime}}{CAf} \cdot {Vref}}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

That is, although the output voltage Vout is supposed to be 0 V, thereis caused an offset voltage as represented by the equation 6. When theoffset voltage is caused, accuracy in detection of the touch position isreduced.

With the signal processing circuit according to the embodiment of thisinvention, the offset voltage can be compensated by adjusting thecapacitances CA3 and CA4 of the third and fourth capacitors C3 and C4 asdescribed above. That is, the offset voltage can be made 0 V byadjusting the capacitances so as to be CA3−CA4=ΔC′ in this case.

For example, when CA1=C+½ ΔC and CA2=C−½ ΔC in the initial state, theoffset voltage can be made 0 V by adjusting the capacitances so as to beCA3=C+½ ΔC and CA4=C−½ ΔC. That is, when CA1 is larger than CA2, it isgood enough that CA3 is adjusted to be larger than CA4 by the differencebetween CA1 and CA2.

The equation 6 holds in this case also, and when the finger 20 of theoperator approaches the touch pad 13 or 14 to cause the capacitancedifference ΔC between CA1 and CA2, the output voltage Vout of theelectric charge amplifier 17 varies in proportion to the capacitancedifference ΔC between the capacitances CA1 and CA2.

Principles of the operations of the touch sensor are hereafter explainedreferring to FIGS. 1A, 1B and 1C. In the following explanation, theoffset voltage is adjusted to 0 V as described above. The explanation isgiven based on a dielectric model in which the finger 20 of the operatoris regarded as a dielectric that is electrically floating.

First, when the finger 20 of the operator approaches the first touch pad13 as shown in FIG. 1A, an electric field between the first touch pad 13and the excitation pad 12 is varied so that the capacitance CA1 of thecapacitor C1 formed between the first touch pad 13 and the excitationpad 12 becomes larger compared with the capacitance CA2 (CA1>CA2). Thisis because the number of lines of electric force originating from theexcitation pad 12 and terminating at the first touch pad 13 is increasedby that the finger 20 of the operator approaches the first touch pad 13.In this case, the output voltage Vout of the electric charge amplifier17 becomes positive (+) as derived from the equation 5. The same resultis obtained when a dielectric material such as an eraser approaches thefirst touch pad 13 instead of the finger 20 of the operator.

When the finger 20 of the operator is placed right above the excitationpad 12 as shown in FIG. 1B, the capacitance CA1 and the capacitance CA2are equal to each other (CA1=CA2). In this case, the output voltage Voutof the electric charge amplifier 17 becomes 0 V.

When the finger 20 of the operator approaches the second touch pad 14 asshown in FIG. 1C, an electric field between the second touch pad 14 andthe excitation pad 12 is varied so that the capacitance CA2 of thecapacitor C2 formed between the second touch pad 14 and the excitationpad 12 becomes larger compared with the capacitance CA1 (CA2>CA1). Inthis case, the output voltage Vout of the electric charge amplifier 17becomes negative (−) as derived from the equation 5.

The touch sensor described above can be used as an ON/OFF switch, sincethe output voltage Vout of the electric charge amplifier 17 is turned tothe positive (+) voltage when the finger 20 of the operator approachesthe first touch pad 13. Besides, the output voltage Vout of the electriccharge amplifier 17 varies linearly with AC. That is, the closer thefinger 20 of the operator approaches to the first touch pad 13, thelarger becomes the positive output voltage Vout. Conversely, the closerthe finger 20 of the operator approaches to the second touch pad 14, thelarger becomes an absolute value of the negative output voltage Vout.Therefore, linear detection (analog detection) of the touch position ofthe finger 20 of the operator is made possible by utilizing thecharacteristics described above.

In addition, noise immunity can be improved with the touch sensordescribed above, since differential capacitance detection is adopted.That is, when a noise is applied to the first and second touch pads 13and 14, an influence of the noise on the first touch pad 13 and aninfluence of the noise on the second touch pad 14 are cancelled out byeach other so that the influence of the noise is suppressed fromappearing in the output voltage Vout of the electric charge amplifier17. Furthermore, since there is no influence of parasitic capacitancesof the first and second touch pads 13 and 14 and the wirings 15, 18 and19, no restriction is imposed on patterning of the touch pads and thelike, enabling arbitrary patterning. Above explanation is based on thedielectric model in which the finger 20 of the operator is regarded as adielectric material. When the finger 20 of the operator is grounded, onthe other hand, an electric field shielding model applies. In this case,the finger 20 of the operator serves to shield the electric field sothat the relative magnitude of the capacitance CA1 to the capacitanceCA2 is reversed. That is, in the electric field shielding model, whenthe finger 20 of the operator approaches the first touch pad 13, thenumber of the lines of electric force originating from the excitationpad 12 and terminating at the first touch pad 13 is decreased because apart of the lines of electric force originating from the excitation pad12 terminates at the finger 20 of the operator. As a result, thecapacitance CA1 becomes smaller compared with the capacitance CA2(CA1<CA2). Which of the dielectric model and the electric fieldshielding model applies is determined depending on the electrical stateof the finger 20 of the operator or its alternative (a pen, an eraser orthe like). However, there is no difference in that the touch positioncan be detected based on the changes in the capacitances, since onlydifference in the case where the electric field shielding model appliesis that the relative magnitude of capacitance CA2 to the capacitance CA1is reversed. The signal processing circuits according to the embodimentsof this invention described below are explained based on the dielectricmodel.

Next, a signal processing circuit according to a second embodiment ofthis invention is explained. The signal processing circuit according tothe second embodiment is a touch sensor that enables detecting eight ormore than eight touch positions on a touch panel with four inputs.

First, a structure of the touch panel is described referring to FIG. 5.Four kinds of touch pads (electrodes) 1-4, that are a first kind, asecond kind, a third kind and a fourth kind of touch pads, are providedon a substrate 30 such as a PCB substrate. Pairs of touch pads made ofone or two kinds of touch pads selected out of the first through fourthkinds of touch pads 1-4 are arrayed in a ring form.

A first through eighth pairs of touch pads (1, 1), (1, 3), (3, 3), (3,2), (2, 2), (2, 4), (4, 4), and (4, 1) are disposed clockwise in anexample shown in FIG. 5. The first pair (1, 1) represents a paircomposed of the first kind of touch pad 1 and another first kind oftouch pad 1, while the second pair (1, 3) represents a pair composed ofthe first kind of touch pad 1 and the third kind of touch pad 3. Each ofthe rest of the pairs represents the similar structure. The pairs oftouch pads include pairs of touch pads composed of a single kind oftouch pads such as (1, 1), (2, 2), (3, 3) and (4, 4).

Each of the pairs of touch pads composed of a single kind of tough padsare depicted as two separate touch pads in the example shown in FIG. 5so that they have the same shape as the pairs of touch pads composed oftwo kinds of touch pads. However, the pairs of touch pads composed of asingle kind of touch pads may be shaped in a single touch pad. Forexample, the first pair of touch pads (1, 1) may be composed of a singlefirst kind of touch pad 1.

Each of the first through eighth pairs of touch pads corresponds to eachof first through eighth locations P1-P8, respectively. An excitation pad(electrode) 31 is placed between neighboring pairs of touch pads. Theexcitation pad 31 corresponds to the excitation pad 12 shown in FIG. 1Ato FIG. 1C. The touch pads of the same kind, for example, four firstkind of touch pads 1, are connected together with a wiring. Each of thesame kinds of touch pads is connected to corresponding each of firstthrough fourth output terminals CO1-CO4, respectively. The excitationpad 31 is connected to an excitation terminal EXC through a wiring.

Next, a structure of the signal processing circuit is explained. Analternating current power supply 32 is connected to the excitationterminal EXC and the excitation pad 31 is provided with an alternatingvoltage. Each of the first through fourth output terminals CO1-CO4 isconnected to corresponding each of four input terminals of a selectioncircuit 33. The selection circuit 33 selects either a signal from acombination of the first and second kinds of touch pads 1 and 2outputted through the first and second output terminals CO1 and CO2 or asignal from a combination of the third and fourth kinds of touch pads 3and 4 outputted through the third and fourth output terminals CO3 andCO4.

An electric charge amplifier 34 is provided in a stage subsequent to theselection circuit 33. The electric charge amplifier 34 is identical tothe electric charge amplifier 17. That is, the electric charge amplifier34 generates a voltage corresponding to a difference between acapacitance of a first capacitor formed between the excitation pad 31and a touch pad of one kind out of the combination selected by theselection circuit 33 and a capacitance of a second capacitor formedbetween the excitation pad 31 and a touch pad of another kind out of thecombination selected by the selection circuit 33.

That is, when the combination of the first and second kinds of touchpads 1 and 2 is selected, the electric charge amplifier 34 generates afirst output voltage V1 corresponding to a difference between acapacitance of a first capacitor formed between the excitation pad 31and the first kind of touch pad 1 and a capacitance of a secondcapacitor formed between the excitation pad 31 and the second kind oftouch pad 2. The capacitance of the first capacitor and the capacitanceof the second capacitor are set to be equal to each other in the initialstate in which the finger of the operator is far away from any of thetouch pads.

Similarly, when the combination of the third and fourth kinds of touchpads 3 and 4 is selected, the electric charge amplifier 34 generates asecond output voltage V2 corresponding to a difference between acapacitance of a third capacitor formed between the excitation pad 31and the third kind of touch pad 3 and a capacitance of a fourthcapacitor formed between the excitation pad 31 and the fourth kind oftouch pad 4. The capacitance of the third capacitor and the capacitanceof the fourth capacitor are set to be equal to each other in the initialstate in which the finger of the operator is far away from any of thetouch pads.

Operations of the touch sensor described above are explained referringto Table 1.

TABLE 1 Output of Electric Output of Electric Charge Amplifier ChargeAmplifier Location Pair of Touch Pads (in Phase 1) (in Phase 2) P1 11 +0 P2 13 + + P3 33 0 + P4 32 − + P5 22 − 0 P6 24 − − P7 44 0 − P8 41 + −

The selection circuit 33 is controlled by a control circuit so that itselects the combination of the first and second kinds of touch pads 1and 2 in a first phase (phase 1) and selects the combination of thethird and fourth kinds of touch pads 3 and 4 in a subsequent phase(phase 2). When the finger of the operator touches the first locationP1, that is a center of the first pair of touch pads (1, 1), thecombination of the first and second kinds of touch pads 1 and 2 isselected in the phase 1. According to the principle described above, theelectric charge amplifier 34 outputs a positive (+) voltage because thecapacitance of the capacitor formed between the excitation pad 31 andthe first kind of touch pad 1 is increased.

In the phase 2, on the other hand, the combination of the third andfourth kinds of touch pads 3 and 4 is selected. In this case, theelectric charge amplifier 34 outputs 0 V. This is because the finger ofthe operator touches the first pair of touch pads (1, 1) and there is nodifference between the capacitance of the capacitor formed between theexcitation pad 31 and the third kind of touch pad 3 and the capacitanceof the capacitor formed between the excitation pad 31 and the fourthkind of touch pad 4. Therefore, the output of the electric chargeamplifier 34 in the phase 1 and in the phase 2 is represented as (+, 0).

Next, when the finger of the operator touches the second location P2,that is a center of the second pair of touch pads (1, 3), thecapacitance of the capacitor formed between the excitation pad 31 andthe first kind of touch pad 1 increases in the phase 1, so that theelectric charge amplifier 34 outputs the positive (+) voltage. Theelectric charge amplifier 34 also outputs the positive (+) voltage inthe phase 2 because the capacitance of the capacitor formed between theexcitation pad 31 and the third kind of touch pad 3 is increased.Therefore, the output of the electric charge amplifier 34 in the phase 1and in the phase 2 is represented as (+, +).

Next, when the finger of the operator touches the third location P3,that is a center of the third pair of touch pads (3, 3), the electriccharge amplifier 34 outputs 0 V in the phase 1 because there is nodifference between corresponding capacitances. The electric chargeamplifier 34 also outputs the positive (+) voltage in the phase 2because the capacitance of the capacitor formed between the excitationpad 31 and the third kind of touch pad 3 is increased. Therefore, theoutput of the electric charge amplifier 34 in the phase 1 and in thephase 2 is represented as (0, +).

When the finger of the operator touches the fourth location P4, that isa center of the fourth pair of touch pads (3, 2), the capacitance of thecapacitor formed between the excitation pad 31 and the second kind oftouch pad 2 increases in the phase 1, so that the electric chargeamplifier 34 outputs a negative (−) voltage. The electric chargeamplifier 34 outputs a positive (+) voltage in the phase 2 because thecapacitance of the capacitor formed between the excitation pad 31 andthe third kind of touch pad 3 is increased. Therefore, the output of theelectric charge amplifier 34 in the phase 1 and in the phase 2 isrepresented as (−, +).

With similar consideration to those as described above, the output ofthe electric charge amplifier 34 in the phase 1 and in the phase 2 is(−, 0) when the finger of the operator touches the fifth location P5,that is a center of the fifth pair of touch pads (2, 2). When the fingerof the operator touches the sixth location P6, that is a center of thesixth pair of touch pads (2, 4), the output of the electric chargeamplifier 34 in the phase 1 and in the phase 2 becomes (−, −). When thefinger of the operator touches the seventh location P7, that is a centerof the seventh pair of touch pads (4, 4), the output of the electriccharge amplifier 34 in the phase 1 and in the phase 2 becomes (0, −).When the finger of the operator touches the eighth location P8, that isa center of the eighth pair of touch pads (4, 1), the output of theelectric charge amplifier 34 in the phase 1 and in the phase 2 becomes(+, −).

The eight locations can be detected by the outputs of the electriccharge amplifier 34 in the phase 1 and in the phase 2 as describedabove. In other words, the eight locations can be detected with the fourinputs CO1-CO4. As a result, the number of input terminals and thenumber of wirings can be substantially reduced compared with theconventional touch sensor. Note that pairs of touch pads (1, 2) and (3,4) are not used because the output of the electric charge amplifier 34is undefined in these cases and these pairs of touch pads do notfunction in the differential capacitance detection.

Only three values +, 0 and − are used as the output of the electriccharge amplifier 34 in detecting the eight locations as described above.Since the electric charge amplifier 34 outputs the analog voltagecorresponding to the capacitance difference ΔC, it is possible to detectmore than eight locations when the analog voltage is used.

The detection of more than eight locations will be explained hereinafterin detail. A line connecting the center of the first pair of touch pads(1, 1) and the center of the fifth pair of touch pads (2, 2) is referredto as a y-axis, as shown in FIG. 5. A line connecting the center of thethird pair of touch pads (3, 3) and the center of the seventh pair oftouch pads (4, 4) is referred to as an x-axis. The x-axis and the y-axisintersect orthogonally. Now an angle formed by the y-axis and a linepointing from an intersection of the x-axis and the y-axis to the touchposition of the finger of the operator is denoted by θ. The angle θ ispositive when it represents a clockwise rotation from the y-axis, and isnegative when it represents a counterclockwise rotation from the y-axis.The angle θ defined as described above is referred to as a touchposition angle. The electric charge amplifier 34 outputs the firstoutput voltage V1 in the phase 1, and outputs the second output voltageV2 in the phase 2.

Then, the first and second output voltages V1 and V2 of the electriccharge amplifier 34 vary continuously in accordance with the change inthe touch position angle θ, as shown in FIGS. 6 and 7. In this case, thefirst output voltage V1 is approximated by cos θ. On the other hand, thesecond output voltage V2 is approximated by sin θ. Here, amplitudes ofthe output voltages V1 and V2 (coefficients of cos θ and sin θ) arenormalized to “1”.

For example, a case where θ=0° corresponds to the first location P1 andthe output voltages are represented as (V1, V2)=(1, 0). A case whereθ=45° corresponds to the second location P2 and the output voltages arerepresented as (V1, V2)=(1/√2, 1/√2). A case where θ=90° corresponds tothe third location P3 and the output voltages are represented as (V1,V2)=(0, 1). A case where θ=−45° corresponds to the eighth location P8and the output voltages are represented as (V1, V2)=(1/√2, −1/√2).

The touch position angle θ can be calculated from the output voltages V1and V2 based on the correlation as described above. In order tocalculate the touch position angle θ more efficiently, it is preferableto use V2/V1 that is a ratio of the output voltage V2 to the outputvoltage V1. V2/V1 is approximated by tan θ. That is, V2/V1=tan θ. Usingan inverse function of tan, the touch position angle θ is represented asθ=arctan (V2/V1).

As understood from FIG. 6, it is not possible to determine the touchposition angle θ uniquely by a value of V2/V1. For example, the value ofV1/V2=tan θ is “1” both at the second location P2 (θ=45°) and at thesixth location P6 (θ=−135°). However, the touch position angle θ can beuniquely determined when the polarities (+, −) of the first and secondoutput voltages V1 and V2 are taken into consideration. For example,both of the output voltages V1 and V2 are positive (+) in the polarityat the second location P2. On the other hand, both of the outputvoltages V1 and V2 are negative (−) in the polarity at the sixthlocation P6 (Refer to FIG. 7.).

That is, in which of four quadrants in FIG. 7 the touch position angle θis located can be determined from the polarities of the first and secondoutput voltages V1 and V2. When (V1, V2)=(+, +), <θ<90° holds. When (V1,V2)=(−, +), 90°<θ<180° holds. When (V1, V2)=(+, −), −90°<θ<0° holds.When (V1, V2)=(−, −), −180°<θ<−90° holds. Therefore, when a range of thetouch position angle θ is determined based on the first and secondoutput voltages V1 and V2, the touch position angle θ can be uniquelydetermined from the equation θ=arctan (V2/V1).

It is preferable that the algorithm to calculate θ from the equationθ=arctan (V2/V1) and the polarities of V1 and V2 as described above isexecuted by an computing unit such as a microcomputer after convertingthe analog values of the output voltages V1 and V2 of the electriccharge amplifier 34 into the digital values by an A/D converter. In thiscase, detection accuracy of the touch position angle θ depends onresolution of the A/D converter.

While the pairs of touch pads (1, 1), (1, 3), (3, 3), (3, 2), (2, 2),(2, 4), (4, 4), and (4, 1) are arrayed in a ring form in the signalprocessing circuit according to the second embodiment, identical pairsof touch pads (1, 1), (1, 3), (3, 3), (3, 2), (2, 2), (2, 4), (4, 4),and (4, 1) are arrayed in line in the same order as shown in FIG. 8 in asignal processing circuit according to a third embodiment of thisinvention. The rest of the structure is the same as in the secondembodiment and its operations are also the same as in the secondembodiment.

With the signal processing circuit according to the third embodiment,the touch positions can be found also from the first and second outputvoltages V1 and V2 of the electric charge amplifier 34. That is, takingthe first location P1, that is a center of the first pair of touch pads(1, 1) as an origin (z=0), a coordinate of a touch position on a linedirecting from the origin toward the second location P2 is representedas z. When P1-P8 are equally spaced at intervals of p, for example, thefirst output voltage V1 is approximated by cos(πz/4p). On the otherhand, the second output voltage V2 is approximated by sin(πz/4p).Therefore, the touch position coordinate z can be calculated from z=4p/πarctan (V2/V1) and the polarities of (V1, V2) as in the secondembodiment.

An example of a structure of a touch sensor system is describedhereafter.

The touch sensor system is formed to include a signal processing circuit50, a touch panel 51 and a microcomputer 52, as shown in FIG. 9. Thetouch panel 51 is identical to the touch panel shown in FIG. 5 or inFIG. 8. The signal processing circuit 50 is structured to include theselection circuit 33, the electric charge amplifier 34, the 16-bit deltasigma A/D converter 35, a drive circuit 36, a control circuit 37, an I²Cinterface circuit 38, a reference voltage generation circuit 39, apower-on reset circuit 40, an oscillator 41, an offset adjustmentcircuit 42, an EEPROM 43, first through fourth input terminalsCIN1-CIN4, a drive terminal CDRV, a serial clock terminal SCL, a serialdata terminal SDA, a power supply terminal VDD, a ground terminal VSSand an interrupt terminal INT.

Each of the signals from the first through fourth output terminalsCO1-CO4 in the touch panel 51 is inputted to each of the first throughfourth input terminals CIN1-CIN4, respectively. The selection circuit 33selects between the combination of the first and second input terminalsCIN1 and CIN2 and the combination of the third and fourth inputterminals CIN3 and CIN4.

Outputs of the selection circuit 33 are inputted to the electric chargeamplifier 34. The electric charge amplifier 34 outputs the first andsecond output voltages V1 and V2. The delta sigma A/D converter 35converts the first and second output voltages V1 and V2 into digitalvalues. The control circuit 37 controls whole signal processing circuit50. Output data of the delta sigma A/D converter 35 is converted intoserial data in a predetermined format by the control circuit 37 and theI²C interface circuit 38, and transmitted to the microcomputer 52through the serial clock terminal SCL and the serial data terminal SDA.

In this case, the output of the delta sigma A/D converter 35 is seriallytransmitted to the microcomputer 52 in synchronization with the serialclock. A program corresponding to the algorithm to calculate the touchposition angle θ or the touch position coordinate z on the touch panel51 as explained in the second or third embodiment is stored in a ROM(Read Only Memory) in the microcomputer 52. The microcomputer 52calculates the touch position angle θ or the touch position coordinate zon the touch panel 51 by executing the program.

The drive circuit 36 is structured to include the alternating currentpower supply 32, and provides the excitation terminal EXC in the touchpanel 51 with the alternating voltage through the drive terminal CDRV.The reference voltage generation circuit 39 generates the referencevoltage Vref that is used in the drive circuit 36.

The power-on reset circuit 40 resets the system when the power supply isturned on. The oscillator 41 generates a system clock. The offsetadjustment circuit 42 compensates the offset in the output voltages V1and V2 of the electric charge amplifier 34 as described above. That is,the offset adjustment circuit 42 determines the adjustment signals basedon the output voltages V1 and V2 of the electric charge amplifier 34 sothat the offset becomes the desired value that is preferably the minimumvalue. In this case, the offset adjustment circuit 42 may determines theadjustment signals based on the output voltages V1 and V2 of theelectric charge amplifier 34 converted into the digital values by thedelta sigma A/D converter 35 so that the offset becomes the desiredvalue that is preferably the minimum value.

The adjustment signals determined by the offset adjustment circuit 42are written into the EEPROM 43 by the control circuit 37 and storedthere. The determined adjustment signals are not erased even when thepower supply of the system is turned off, since the EEPROM 43 is anon-volatile memory.

When the power supply of the system is turned on again, the controlcircuit 37 reads the adjustment signals out of the EEPROM 43 and setsthem in the offset adjustment circuit 42. In this case, the offsetadjustment circuit 42 has a register that stores the adjustment signalstemporarily. With this, once the offset in the output voltage Vout ofthe electric charge amplifier 34 is adjusted, the offset in the outputvoltage Vout of the electric charge amplifier 34 is automaticallyadjusted every time the power supply of the system is turned on.

The noise immunity can be improved with the signal processing circuitfor the electrostatic capacity type touch sensor according to theembodiments of this invention, since the differential capacitancedetection is adopted. In addition, the accuracy in the detection of thetouch position can be improved by adjusting the offset in the outputvoltage of the electric charge amplifier.

1. A signal processing circuit for an electrostatic capacity type touchsensor that receives a signal from a first and second touch padsdisposed on a touch panel using an excitation pad disposed between thefirst and second touch pads for signal processing, the signal processingcircuit comprising: a first alternating current power supply generatinga first alternating voltage; an electric charge amplifier generating anoutput voltage corresponding to a difference between a first capacitanceof a first capacitor and a second capacitance of a second capacitor whenthe first alternating voltage is applied to the excitation pad, thefirst capacitor being formed between the first touch pad and theexcitation pad, and the second capacitor being formed between the secondtouch pad and the excitation pad; and an offset adjustment circuitadjusting an offset in the output voltage of the electric chargeamplifier.
 2. The signal processing circuit of claim 1, furthercomprising a third capacitor connected in series with the firstcapacitor and having a variable capacitance, a fourth capacitorconnected in series with the second capacitor and having a variablecapacitance, and a second alternating current power supply generating asecond alternating voltage that is opposite in phase to the firstalternating voltage, wherein the offset adjustment circuit adjusts thethird capacitance or the fourth capacitance in response to the outputvoltage of the electric charge amplifier when the second alternatingvoltage is applied to the third capacitor or the fourth capacitor. 3.The signal processing circuit of claim 1, wherein the third capacitorcomprises a plurality of capacitors and a first switching circuit, thefirst switching circuit connecting one of the capacitors selected by anadjustment signal from the offset adjustment circuit so that theselected capacitor connects the first capacitor and the secondalternating current power supply, and wherein the fourth capacitorcomprises a plurality of capacitors and a second switching circuit, thesecond switching circuit connecting one of the capacitors selected by anadjustment signal from the offset adjustment circuit so that theselected capacitor connects the second capacitor and the secondalternating current power supply.
 4. The signal processing circuit ofclaim 3, further comprising an electrically writable/erasablenon-volatile memory, and a control circuit writing the adjustment signalfrom the offset adjustment circuit into the non-volatile memory.
 5. Thesignal processing circuit of claim 4, wherein the control circuit readsthe adjustment signal out of the non-volatile memory and sets theadjustment signal into the offset adjustment circuit.
 6. A signalprocessing device comprising: a touch panel comprising, a plurality ofpairs of touch pads, each of the touch pads being a first kind of touchpad, a second kind of touch pad, a third kind of touch pad or a fourthkind of touch pad, and each of the pairs of touch pads comprising twotouch pads of a single kind or two touch pads of two different kinds,and an excitation pad disposed between neighboring two of the pairs oftouch pads; and a signal processing circuit for the touch panelcomprising, a first alternating current power supply providing theexcitation pad with a first alternating voltage, an electric chargeamplifier generating a first output voltage corresponding to adifference between a capacitance of a first capacitor formed between theexcitation pad and the first kind of touch pad and a capacitance of asecond capacitor formed between the excitation pad and the second kindof touch pad and generating a second output voltage corresponding to adifference between a capacitance of a third capacitor formed between theexcitation pad and the third kind of touch pad and a capacitance of afourth capacitor formed between the excitation pad and the fourth kindof touch pad, and an offset adjustment circuit adjusting an offset inthe first output voltage or an offset in the second output voltage. 7.The signal processing device of claim 6, further comprising a fifthcapacitor connected in series with the first capacitor and having avariable capacitance, a sixth capacitor connected in series with thesecond capacitor and having a variable capacitance, and a secondalternating current power supply generating a second alternating voltagethat is opposite in phase to the first alternating voltage, wherein theoffset adjustment circuit adjusts the fifth capacitance or the sixthcapacitance in response to the first or second output voltage when thesecond alternating voltage is applied to the fifth capacitor or thesixth capacitor.
 8. The signal processing device of claim 6, wherein thefifth capacitor comprises a plurality of capacitors and a firstswitching circuit, the first switching circuit connecting one of thecapacitors selected by an adjustment signal from the offset adjustmentcircuit so that the selected capacitor is connected between the firstcapacitor and the second alternating current power supply, and whereinthe sixth capacitor comprises a plurality of capacitors and a secondswitching circuit, the second switching circuit connecting one of thecapacitors selected by an adjustment signal from the offset adjustmentcircuit so that the selected capacitor is connected between the secondcapacitor and the second alternating current power supply.
 9. The signalprocessing device of claim 8, further comprising an electricallywritable/erasable non-volatile memory, and a control circuit writing theadjustment signal from the offset adjustment circuit into thenon-volatile memory.
 10. The signal processing device of claim 9,wherein the control circuit reads the adjustment signal out of thenon-volatile memory and sets the adjustment signal into the offsetadjustment circuit.